1. Field of the Invention
This invention relates to an image recording apparatus which records image or character data on a recording medium, such as paper or the like, and especially relates to a recording apparatus which receives image data in the form of an electric image signal from an external apparatus, e.g., a host computer, an image reader, a controller or the like, and outputs high quality visible images.
2. Background
In one type of conventional image recording apparatus (for convenience, generically called a "printer" in this specification), e.g., a laser beam printer which applies electrostatic photography, or the like, generally a host computer (as an example of what is herein termed "external apparatus") executes image processing, e.g., a screen process or a dither process and binarizes image data prior to supplying the data to the printer, to obtain halftone images. In another conventional laser beam printer, disclosed in U.S. Pat. No. 4,800,442, a turning-on period of a laser, is controlled by controlling the width of a pulse, in the main scanning direction, representing the image. To express halftone images, the turning-on period of the laser is shortened compared to the turning-on period which would correspond to one image dot (i.e., pure black).
Thus, it is convenient for the host computer to carry out the various kinds of image processes, from the viewpoint of data storing ability and the like. Further, the host computer can send a binary signal to a printer, to lessen the amount of data to be sent, and it is effective for such sending of data.
However, the above-mentioned conventional printers generally have different halftone characteristics from each other depending on types and devices of printers, so that those printers output images having different densities in a given portion even if the same image data is supplied to these printers. For example, the external apparatus supplies a 4.times.4 dither pattern (an example is shown in FIG. 2) to printers, one of which has an input/output characteristic A (shown as a solid line in FIG. 3), and another of which has another input/output characteristic B (shown as a broken line in FIG. 3). As a result, even if the same image signal, processed with the same dither pattern, is supplied, it can occur that one printer may be unable to express a bright area because a small point is not recorded, while another printer cannot express a dark area because a small white point is covered by other, black marks.
Strictly speaking, not only a halftone image like a photograph but a character image also has the same problem as discussed above. Especially a character image which has small characters expressed with thin lines has this problem because the thin lines are output with lines of different widths by different printers.
Thus, when an image processing system is structured with one host computer and several kinds of printers, there are problems such as an output image on a recording medium, e.g., a paper, being totally blurred and having thin-lined characters, or another output image being totally dark and has crushed characters.